The present invention relates to a vacuum tube with an entrance window for transmitting radiation such as an X-ray image intensifier tube, X-ray tube, radiation detector, betatron doughnut tube, etc., and a method for manufacturing the same.
Conventionally, vacuum tubes of this type have an entrance window through which radiation is passed and have a vacuum-tight structure such that a vacuum or a predetermined gas atmosphere is kept therein. In an X-ray image intensifier tube, for example, its entrance window has a large diameter of approximately 150 to 400 mm, and X-rays transmitted through an object of detection are introduced through the entrance window. The entrance window is formed not of glass whose rates of X-ray absorption and scattering are relatively high, but of aluminum or titanium. In an X-ray tube using a metal as a material for its central portion, an exit window needs to be located very close to an anode target, which is liable to be increased in temperature and to emit secondary electrons. Therefore, the exit window must withstand a very high temperature. Under this condition, titanium or a titanium-base alloy (all of these materials are hereinafter referred to as titanium-base material) is practically used as a metal material which has low rates of radiation absorption and scattering and high resistance against atmospheric pressure.
The vacuum tubes of this type, however, partially consist of an insulator such as glass or ceramics. The insulator is used because it is necessary that various internal electrodes be supported in the vacuum tubes, lead wires be led out of the vacuum tubes, and visible light images be transmitted to the outside of the vacuum tubes. Even if the insulator is not used for any part of the vacuum tubes, they are seldom formed of titanium-base material only. In general, the vacuum tubes are formed by joining a titanium-base material, and the vacuum tubes have an envelope which is formed by another metal material in a vacuum-tight manner at least at one portion thereof. Kovar (trademark), stainless steel, iron as a high-permeability material, or an alloy containing iron (all of these materials are hereinafter referred to as iron-base material) is often used for the metal material. The iron-base material can stably be joined with glass or ceramics.
A method of joining titanium-base material to iron-base material in a vacuum-tight manner in one such conventional vacuum tube is disclosed in Japanese Patent Disclosure No. 3340/82.
The vacuum tube manufactured by this conventional vacuum-tight welding method comprises an entrance window member for radiation transmission and an entrance window supporting frame joined to the peripheral edge portion of the entrance window member in a vacuum-tight manner. The entrance window member is formed of a titanium-base material, while the entrance window supporting frame is formed of an iron-base material. An intermediate member is interposed between the peripheral edge portion of the entrance window member and the entrance window supporting frame. The intermediate member is formed of silver solder, gold solder or other metal material which melts at a temperature lower than the critical temperature of the entrance window member. The entrance window member and the entrance window supporting frame are joined together by spot welding through the medium of the intermediate member.
According to this prior art joining method, the intermediate member is interposed between the entrance window member and the entrance window supporting frame. A composite structure as an object of welding consisting of the entrance window member, the intermediate member, and the entrance window supporting frame is inserted between a pair of electrodes of a spot welding machine. A pressure ranging from 40 to 200 kg/cm.sup.2 and a pulse current of 5,000 to 40,000 A/cm.sup.2 are applied between the two electrodes. During intermission of the conduction, the joint of the object of welding is moved a given distance along the periphery of the entrance window member for spot welding. Thus, the entrance window supporting frame and the entrance window member, along with the intermediate member between them, are joined in a vacuum-tight manner along the whole perimeter of the window.
The prior art vacuum-tight joining method, however, involves various problems related to a tack welding step. If the entrance window member is formed of a flat titanium-base material, it needs to be previously tacked to the entrance window supporting member before a final welding step. This is so because the flat titanium-base material, without the tack welding steps, would be bent by thermal expansion during the final welding process. For example, the tack welding is either diagonally performed at four corners or alternately performed at 16 spots which include the four corner portions of the entrance window member and are arranged at regular intervals along its periphery. After the tack welding step, the final welding is conducted to cover the whole perimeter of the window including the tacked spots. The intermediate member is oxidized due to the tack-welding. Therefore, if these tacked regions are welded again by the final spot welding, they can provide less weld strength between the entrance window member and the entrance window supporting frame than the non-tacked regions between them. Thus, the vacuum-tight structure of the vacuum tube is lowered in reliability. Moreover, the intermediate member is melted by the tack welding, so that it will lack or become too thin in some positions to undergo the final spot welding. Accordingly, the tacked regions are different in welding conditions from the non-tacked regions. The difference in welding conditions causes splashes on either side of the intermediate member, which will be scattered into the vacuum tube to soil it.